The present invention relates to an ignition device for internal combustion engine, having a function of generating a spark discharge between electrodes of a spark plug by applying an igniting high voltage generated in an ignition coil between the electrodes of the spark plug, and a function of generating an ionic current after completion of the spark discharge.
In an internal combustion engine used as a car engine or the like, when an air-fuel mixture is burned by a spark discharge in a spark plug, ions are produced with the combustion of the air-fuel mixture. Therefore, if a voltage is applied between electrodes of the spark plug after the air-fuel mixture is burned by the spark discharge of the spark plug, an ionic current flows. Because the amount of produced ions varies in accordance with the state of combustion of the air-fuel mixture, ignition failure, knocking or the like can be detected if the ionic current is detected and analyzed.
As an example of a related-art ignition device for internal combustion engine having a function of generating such an ionic current, there is a device in which a center electrode 61 of a spark plug 13 is electrically connected to one end of a secondary winding 34 of an ignition coil 15 while a capacitor 45 is series-connected to the other end of the secondary winding 34 as shown in FIG. 4. The ignition device 101 for internal combustion engine is configured so that the capacitor 45 is charged by a discharge current 22 (secondary current 22) flowing in the secondary winding 34 of the ignition coil 15 and the spark plug 13 at the time of generation of a spark discharge in the spark plug 13, and so that the charged capacitor 45 is discharged after completion of the spark discharge to thereby apply a voltage between electrodes of the spark plug 13 through the secondary winding 34 to generate an ionic current 42. Further, a detection resistor 47 is provided at the other end of the capacitor 45 opposite to the secondary winding 34 so that the ionic current is detected on the basis of the voltage between opposite ends of the detection resistor 47.
Incidentally, in the ignition device 101 for internal combustion engine, a Zener diode 111 is provided in parallel to the capacitor 45 to prevent the capacitor 45 from being broken by overcharge and to limit the voltage between the opposite ends of the capacitor 45 to a constant value (100 to 300 V).
As described above, in the ignition device for internal combustion engine using the capacitor 45 as a power supply for detecting an ionic current, it is unnecessary to provide any special power supply unit (such as a battery) exclusively used for detecting an ionic current. Hence, there is an advantage that a relatively small number of parts can be used while the size of the ignition device can be reduced.
In the ignition device 101 for internal combustion engine, however, magnetic flux energy is stored in the ignition coil 15. For this reason, a voltage (several kV) reversed in polarity to an igniting high voltage is generated in the secondary winding 34 when current conduction to a primary winding 33 is started. Hence, there is fear that the spark plug 13 may generate a spark discharge before normal ignition timing to thereby cause wrong ignition of an air-fuel mixture.
FIG. 6 is a time chart showing states of a first command signal and the voltage between the opposite ends of the secondary winding in the ignition device 101 for internal combustion engine shown in FIG. 4. Incidentally, when the level of the first command signal is low, an igniter 17 is open-circuited so that there is no current flowing in the primary winding 33. On the other hand, when the level of the first command signal is high, the igniter 17 is short-circuited so that a current flows in the primary winding 33. In FIG. 6, the waveform of the voltage between the opposite ends of the secondary winding 34 is shown with the igniting high voltage as a negative-polarity voltage. Hence, points of time t12 and t15 show igniting high voltage generation timing (ignition timing).
In FIG. 6, points of time t11 and t14 show start timing for conduction of the primary current. It is found that a voltage (several kV) reversed in polarity to the igniting high voltage is generated between the opposite ends of the secondary winding 34 in this timing. There is fear that wrong ignition may be caused by this voltage.
To prevent the generation of such wrong ignition, in the ignition device 101 for internal combustion engine shown in FIG. 4, for example, a so-called reverse current prevention diode may be provided in a current-conduction path formed between one end of the secondary winding 34 and the spark plug 13 so that a current is allowed to flow in the current-conduction path of the secondary current 22 only at the time of conduction of the primary current 21.
If the reverse current prevention diode is provided in the ignition device 101 for internal combustion engine shown in FIG. 4, it is however impossible to detect an ionic current flowing in between the electrodes of the spark plug 13 because the capacitor 45 can be charged by the secondary current 22 but cannot be discharged due to the reverse current prevention diode.
An ignition device 103 for internal combustion engine shown in FIG. 5 is configured in consideration of this problem. In the ignition device 103, a reverse current prevention diode 31 is provided and an ionic current detection circuit 113 for applying an ionic current-detecting voltage to the spark plug 13 through a current-conduction path different from the secondary winding 34 is provided so that an ionic current can be detected. The ionic current detection circuit 113 is configured as follows. An ionic current-detecting voltage is applied to the spark plug 13 by an internal power supply 115. An ionic current is detected on the basis of the voltage between the opposite ends of the detection resistor 47. A discrimination circuit 55 outputs an ionic current detection result signal 24 to an electronic control unit. Incidentally, an applied voltage-limiting Zener diode 53 prevents a signal of an excessive voltage higher than the allowable maximum input voltage value from being input to the discrimination circuit 55. Hence, the discrimination circuit 55 is prevented from being broken.
In the ignition device 103 for internal combustion engine configured as described above, an inflow prevention diode 117 for preventing the secondary current 22 from flowing into the ionic current detection circuit 113 at the time of generation of the igniting high voltage is provided in order to prevent the ionic current detection circuit 113 from being broken by application of the igniting high voltage. In addition, the inflow prevention diode 117 prevents the secondary current 22 from leaking to the ionic current detection circuit 113. Hence, the inflow prevention diode 117 is also effective in preventing energy supplied to the spark plug 13 from being reduced at the time of generation of the igniting high voltage.
In the ignition device 103 for internal combustion engine shown in FIG. 5, it is however necessary to make the inflow prevention diode 117 from a high-voltage-proof diode of an allowable withstand voltage not lower than the igniting high voltage (about 40 kV) because the inflow prevention diode 117 is connected on the secondary high potential side. At the existing time, it is impossible to obtain such a diode constituted by one high-voltage-proof element.
Therefore, when a plurality of diodes series-connected in order to obtain an allowable withstand voltage not lower than the igniting high voltage as a whole are provided as the inflow prevention diode 117, the ignition device 103 for internal combustion engine shown in FIG. 5 can be achieved.
When such a plurality of diodes series-connected are used, however, the probability that a failure will be included in any one of the diodes becomes high. Hence, there is a problem that reliability is lowered compared with the case where the inflow prevention diode 117 is constituted by one diode. In addition, because the plurality of diodes are used under a particularly severe environment in which a high voltage is applied, there is also a problem that the probability that any one of the diodes will be broken is high.
For this reason, in the ignition device 103 for internal combustion engine shown in FIG. 5, there is fear that the ionic current 42 cannot be detected appropriately because the inflow prevention diode 117 is broken and cannot work normally.
If the ionic current detection circuit is connected to the other end of the secondary winding opposite to the igniting high voltage generation end in order to solve this problem, it is unnecessary to provide any high-voltage-proof diode.
When the ionic current detection circuit is simply connected to the other end of the secondary winding opposite to the igniting high voltage generation end, however, the ionic current-detecting voltage held in the ionic current detection circuit is absorbed to the other end of the secondary winding opposite to the igniting high voltage generation end at the time of generation of the discharge current. As a result, the ionic current-detecting voltage is lowered at the time of detection of the ionic current, so that there is fear that the ionic current cannot be detected appropriately.
Therefore, the invention aims at solving the problems and an object of the invention is to provide an ignition device for internal combustion engine in which wrong ignition of an air-fuel mixture can be restrained from being caused by a spark discharge generated in a spark plug at the time of carrying a current to a primary winding and in which an ionic current between electrodes of the spark plug can be generated and detected.
To achieve the foregoing object, in accordance with the invention, there is provided an ignition device for internal combustion engine having: an ignition coil including a primary winding, and a secondary winding, the ignition coil generating an igniting high voltage in the secondary winding by turning off a primary current flowing in the primary winding; an ignition switching unit for turning on/off the primary current flowing in the primary winding of the ignition coil; and a spark plug connected to an igniting high voltage generation end of the secondary winding for generating a spark discharge between electrodes of the spark plug in the condition that a discharge current generated on the basis of the igniting high voltage flows in the spark plug; the ignition device further having: a reverse current prevention unit series-connected on a current-conduction path of the discharge current connecting the secondary winding to the spark plug, the reverse current prevention unit permitting conduction of the discharge current in the spark plug but preventing conduction of a current generated in the secondary winding at the time of carrying a current to the primary winding; a voltage application unit connected to the other end of the secondary winding opposite to the igniting high voltage generation end for applying an ionic current-detecting voltage to the spark plug, the ionic current-detecting voltage being identical in polarity to the igniting high voltage applied to the spark plug; an ionic current detection unit for detecting an ionic current flowing in between the electrodes of the spark plug on the basis of application of the ionic current-detecting voltage; and an ionic current detection switching unit series-connected on a current-conduction path of the ionic current-detecting voltage connecting the voltage application unit to the other end of the secondary winding for making the current-conduction path non-conductive to apply the ionic current-detecting voltage at the time of generation of the igniting high voltage but making the current-conduction path conductive to apply the ionic current-detecting voltage at the time of detection of the ionic current on the basis of external commands.
That is, in the ignition device for internal combustion engine according to the invention, the reverse current prevention unit is provided on the current-conduction path of the discharge current connecting the secondary winding of the ignition coil to the spark plug so that the direction of the current allowed to be carried by the current-conduction path of the discharge current (secondary current) is limited to one direction. That is, the reverse current prevention unit prevents current conduction from being caused by the voltage (several kV) generated between the opposite ends of the secondary winding at the time of carrying a current to the primary winding, so that a spark discharge is prevented from being generated between the electrodes (center electrode and ground electrode) of the spark plug at the time of carrying a current to the primary winding.
Moreover, in the ignition device for internal combustion engine, the ionic current detection circuit is connected to the other end of the secondary winding opposite to the igniting high voltage generation end. Hence, because the ionic current detection circuit is not influenced by the igniting high voltage, it is unnecessary to provide any high-voltage-proof inflow prevention diode for protecting the ionic current detection circuit.
Moreover, in the ignition device for internal combustion engine, the ionic current detection switching unit is provided as well as the ionic current detection circuit is connected to the other end of the secondary winding opposite to the igniting high voltage generation end. Hence, the ionic current-detecting voltage stored in the voltage application unit can be prevented from being absorbed to the other end of the secondary winding opposite to the igniting high voltage generation end at the time of generation of the igniting high voltage. As a result, the ionic current-detecting voltage required at the time of detection of the ionic current can be applied to the spark plug so that the ionic current can be detected.
Incidentally, for example, the ionic current detection switching unit may be constituted by a switch which is formed so that an internal path of the switch is short-circuited or open-circuited on the basis of commands given from a control unit for controlling the operations of respective parts in the internal combustion engine. That is, the ionic current detection switching unit is formed so that the current-conduction path is made conductive when the ionic current detection switching unit is short-circuited, and that the current-conduction path is made non-conductive when the ionic current detection switching unit is open-circuited.
Moreover, the control unit for drive-controlling the ionic current detection switching unit is provided so that the time zone of making the current-conduction path conductive (i.e., ionic current detection window) can be changed on the basis of the operating state of the internal combustion engine. Hence, the ionic current detection window can be set to be adapted to the operating state of the internal combustion engine. Further, just after completion of the spark discharge, a large amount of noise component is superposed on the ionic current. Therefore, when the ionic current detection window is set so that the noise component can be avoided, the influence of noise is suppressed so that the ionic current can be detected accurately.
Preferably, in the ignition device for internal combustion engine, an auxiliary discharge path-forming unit provided in a position different from a path constituted by the voltage application unit, the ionic current detection unit and the ionic current detection switching unit may be provided as a current-conduction path for a current flowing in the secondary winding at the time of generation of an igniting high voltage. Hence, even in the case where the path constituted by the voltage application unit, the ionic current detection unit and the ionic current detection switching unit is electrically disconnected from the secondary winding by a certain cause, a current-conduction path can be constituted by the auxiliary discharge path-forming unit. Hence, the current-conduction path for the discharge current can be secured.
Incidentally, it is known that when a voltage is applied between electrodes of the spark plug to generate an ionic current, the ionic current which can be generated in the case where the voltage is applied so that the center electrode and the ground electrode are positive and negative respectively in terms of polarity is larger in quantity than the ionic current which can be generated in the case where the voltage is applied so that the center electrode and the ground electrode are negative and positive respectively in terms of polarity. This is because when positive ions large in volume are supplied with electrons from the ground electrode having a surface area larger than that of the center electrode, a larger amount of electrons can be exchanged and transferred.
That is, in the ignition device for internal combustion engine configured as described above, the polarity of the voltage applied to the center electrode of the spark plug by the igniting high voltage is preferably positive. Incidentally, the positive or negative polarity of each end portion of the secondary winding at the time of generation of the igniting high voltage can be set by adjustment of the respective winding directions of the primary and secondary windings in the ignition coil.
Incidentally, the voltage application unit provided in the ignition device for internal combustion engine may have a boosting unit by which a voltage given from an external power supply such as an on-vehicle battery is boosted to a predetermined voltage value required as the ionic current-detecting voltage so that the ionic current-detecting voltage can be output. Or the voltage application unit may be configured so that the ionic current-detecting voltage can be output on the basis of electric energy stored in the inside of the voltage application unit.
Therefore, in the ignition device for internal combustion engine, for example, the voltage application unit may be preferably formed electrically chargeably and dischargeably so that the voltage application unit is electrically charged by an interrupting-time primary induced voltage generated between opposite ends of the primary winding at the time of conduction of the discharge current in the spark plug to thereby apply the ionic current-detecting voltage to the spark plug.
At the time of conduction of the discharge current into the spark plug, an igniting high voltage is induced in the secondary winding and an induced voltage (interruption-time primary induced voltage) is generated in the primary winding by mutual induction. The interruption-time primary induced voltage is not lower than a voltage value (about 100 V to about 300 V) required for generating an ionic current. For this reason, the voltage application unit charged by the interruption-time primary induced voltage can store energy required for generating the ionic current and can output an ionic current-detecting voltage of not lower than the voltage value required for generating the ionic current.
The interruption-time primary induced voltage is also generated as the igniting high voltage to be applied to the spark plug is generated. Hence, because the voltage application unit can be charged by the interruption-time primary induced voltage, it is unnecessary to provide newly any charge voltage supply unit for supplying electric energy to charge the voltage application unit.
In the ignition device for internal combustion engine, for example, the voltage application unit may be preferably formed electrically chargeably and dischargeably so that the voltage application unit is electrically charged by a current-conduction-time secondary induced voltage generated between opposite ends of the secondary winding at the time of current-conduction of the primary winding to thereby apply the ionic current-detecting voltage to the spark plug.
At the time of conduction of the primary current, an induced voltage (current-conduction-time secondary induced voltage) is generated in the secondary winding. The current-conduction-time secondary induced voltage is lower in voltage value than the igniting high voltage but reaches about 2 kV or higher. That is, the current-conduction-time secondary induced voltage is not lower than the voltage value (about 100 V to about 300 V) required for generating the ionic current. Hence, the voltage application unit charged by the current-conduction-time secondary induced voltage can store energy required for generating the ionic current.
The current-conduction-time secondary induced voltage is also generated as conduction of the primary current starts for storing energy required for generating the igniting high voltage in the ignition coil. Hence, because the voltage application unit is charged by the current-conduction-time secondary induced voltage, it is necessary to provide newly any charge voltage supply unit for supplying electric energy to charge the voltage application unit.
In the ignition device for internal combustion engine, for example, the voltage application unit may be preferably formed electrically chargeably and dischargeably so that the voltage application unit is electrically charged by both a current-conduction-time secondary induced voltage generated between opposite ends of the secondary winding at the time of current-conduction of the primary winding and an interrupting-time primary induced voltage generated between opposite ends of the primary winding at the time of conduction of the discharge current in the spark plug to thereby apply the ionic current-detecting voltage to the spark plug.
That is, both current-conduction-time secondary induced voltage and the interruption-time primary induced voltage are used for charging the voltage application unit. Hence, when the voltage application unit is to be charged, energy required for generating the ionic current can be surely stored in the voltage application unit. In addition, it is unnecessary to provide newly any charge voltage supply unit for supplying electric energy to charge the voltage application unit.
Incidentally, as the method for charging the voltage application unit by the current-conduction-time secondary induced voltage, there is, for example, a method in which a current generated on the basis of the current-conduction-time secondary induced voltage is supplied to the voltage application unit through the ionic current detection switching unit. In this method, it is however necessary to execute a drive control process for making the ionic current detection switching unit conductive (short-circuited) in accordance with the charge timing. Hence, there is a problem that the process of controlling the ignition device for internal combustion engine is complicated.
Therefore, preferably, the ignition device for internal combustion engine may further have a charge path-forming unit connected in parallel to the ionic current detection switching unit for preventing conduction of the discharge current but permitting conduction of a current generated on the basis of the current-conduction-time secondary induced voltage, wherein the current generated on the basis of the current-conduction-time secondary induced voltage is supplied to the voltage application unit through the charge path-forming unit to thereby electrically charge the voltage application unit.
The charge path-forming unit can carry a current generated on the basis of the current-conduction-time secondary induced voltage to thereby supply the current to the voltage application unit. That is, because the charge path-forming unit is provided, the voltage application unit can be electrically charged by the current-conduction-time secondary induced voltage without execution of any complex control process for drive-controlling the ionic current detection switching unit in accordance with the charge timing. In addition, because the charge path-forming unit prevents conduction of a current generated in the secondary winding on the basis of the igniting high voltage, the voltage application unit is not influenced by the igniting high voltage.
Incidentally, when the charge path-forming unit is provided, it is preferable to suppress the influence of the igniting high voltage on the charge path-forming unit. Therefore, the charge path-forming unit may be preferably provided in the ignition device for internal combustion engine configured so that the high potential side end portion of the secondary winding at the time of generation of the igniting high voltage is connected to the center electrode of the spark plug through the reverse current prevention unit whereas the low potential side end portion of the secondary winding at the time of generation of the igniting high voltage is connected to the voltage application unit through the ionic current detection switching unit. Hence, the influence of the igniting high voltage on the charge path-forming unit can be suppressed to be small.
In the ignition device for internal combustion engine, for example, the charge path-forming unit may be preferably constituted by a diode.
The charge path-forming unit constituted by a diode is connected in parallel to the ionic current detection switching unit. The charge path-forming unit can prevent conduction of a current generated in the secondary winding on the basis of the igniting high voltage but can permit conduction of a current generated on the basis of the current-conduction-time secondary induced voltage. Hence, a charge path for charging the voltage application unit can be formed.
Incidentally, when a diode is used for permitting a current flowing from the secondary winding into the voltage application unit but preventing a current flowing from the voltage application unit into the secondary winding, the diode may be preferably provided so that an anode of the diode is connected to a junction point between the ionic current detection switching unit and the secondary wiring whereas a cathode of the diode is connected to a junction point between the ionic current detection switching unit and the voltage application unit.
In the ignition device for internal combustion engine, for example, the voltage application unit may be preferably constituted by a capacitor.
That is, because the capacitor is a chargeable and dischargeable capacitance element, the capacitor can be charged by the interruption-time primary induced voltage or the current-conduction-time secondary induced voltage and can output the ionic current-detecting voltage. Hence, when the voltage application unit is constituted by a capacitor, the ionic current-detecting voltage can be applied to the spark plug.
Preferably, the ignition device for internal combustion engine may further have a protection unit for protecting the voltage application unit by limiting the charge voltage of the voltage application unit to be not higher than an allowable maximum charge voltage value.
The provision of the protection unit can prevent the voltage application unit from being overcharged at the time of charging the voltage application unit and can prevent the voltage application unit from being broken due to the overcharging.
Moreover, because the protection unit limits the charge voltage of the voltage application unit to be not higher than the allowable maximum charge voltage value, the charge voltage of the voltage application unit can be kept substantially constant at the allowable maximum charge voltage value. Hence, the ionic current-detecting voltage output from the voltage application unit can be kept substantially constant. In addition, because the ionic current-detecting voltage can be kept substantially constant, the detection value of the ionic current can be prevented from varying in accordance with the change of the voltage value of the ionic current-detecting voltage.
In the ignition device for internal combustion engine, for example, the protection unit may be preferably constituted by a Zener diode.
That is, when the voltage (charge voltage) between the opposite ends of the voltage application unit is not lower than the Zener voltage (break-down voltage) of the Zener diode, a current is carried by the Zener breakdown of the Zener diode. Hence, the charge voltage of the voltage application unit can be limited to be not higher than the allowable maximum charge voltage value, so that the voltage application unit can be protected.
Incidentally, in this case, as the Zener diode, there may be preferably used a Zener diode exhibiting a Zener voltage not higher than the allowable maximum charge voltage value of the voltage application unit.
For example, in order to prevent overcharge to protect the voltage application unit when a current flows from the ionic current detection switching unit into the voltage application unit, the Zener diode may be preferably provided so that a cathode of the Zener diode is connected to an end of the voltage application unit connected to the ionic current detection switching unit whereas an anode of the Zener diode is connected to the other end of the voltage application unit.
Incidentally, when conduction of the discharge current is interrupted with the completion of the spark discharge, magnetic flux density in the ignition coil changes. With the change of magnetic flux density, an induced voltage is generated in the secondary winding. Hence, the secondary winding in which the induced voltage is generated and the stray capacitance of the ionic current-conduction path constitute a resonant circuit, so that voltage-damping oscillation is generated. Hence, when the voltage application unit and the secondary winding are connected to each other in the condition that the resonant circuit is formed, charge stored in the voltage application unit is absorbed to the secondary winding by the influence of the voltage-damping oscillation. As a result, the output voltage of the voltage application unit is reduced. Hence, there is fear that the ionic current-detecting voltage cannot be applied.
Incidentally, such voltage-damping oscillation is not continued for a long time up to the start timing of current conduction into the primary winding in the next combustion cycle after interruption of conduction of the discharge current due to the completion of the spark discharge but is extinguished (converged) after the passage of a predetermined time.
Therefore, preferably, the ignition device for internal combustion engine may further have a detection timing control unit for drive-controlling the ionic current detection switching unit to make the current-conduction path conductive to apply the ionic current-detecting voltage after the passage of a detection delay time required for convergence of voltage-damping oscillation generated on the secondary side of the ignition coil after completion of a spark discharge in the spark plug.
That is, configuration is made so that the ionic current-detecting voltage is applied to the spark plug by drive-controlling the ionic current detection switching unit not just after completion of the spark discharge but after the passage of a detection delay time after the completion of the spark discharge. Because the ionic current detection switching unit is drive-controlled after the passage of the detection delay time after the completion of the spark discharge in this manner, charge stored in the voltage application unit can be prevented from being absorbed to the secondary winding by the influence of the voltage-damping oscillation.
Incidentally, because the voltage-damping oscillation is converged after the passage of a predetermined time after the completion of the spark discharge as described above, the influence of the voltage-damping oscillation can be surely avoided at the time of detection of the ionic current if the detection delay time is set to be not shorter than the time required for convergence of the voltage-damping oscillation.
Moreover, because configuration is made so that the ionic current is detected by applying the ionic current-detecting voltage to the spark plug after the passage of the detection delay time after the completion of the spark discharge, the ionic current can be detected without influence of noise superposed on the ionic current on the basis of generation of the voltage-damping oscillation just after the completion of the spark discharge.
Next, there has been recently discussed a technique in which the ionic current flowing due to ions near to the electrodes of the spark plug just after the completion of the spark discharge generated between the electrodes of the spark plug is used for detecting knocking. If knocking occurs in the internal combustion engine, the air-fuel mixture is compressed by the shock wave of knocking so that the ionic current vibrates. When, for example, the vibration of the ionic current value is not smaller than a predetermined value, a decision can be made that knocking is present. On the other hand, when the vibration of the ionic current value is smaller than the predetermined value, a decision can be made that knocking is absent. Incidentally, there is a knocking generation timing difference between an operating state in which the combustion of the air-fuel mixture progresses slowly (low rotational speed and low load state) and an operating state in which the combustion of the air-fuel mixture progresses rapidly (high rotational speed and high load state). Specifically, the knocking generation timing in an operating state in which the combustion of the air-fuel mixture progresses rapidly is earlier than that in an operating state in which the combustion of the air-fuel mixture progresses slowly.
Therefore, if the spark discharge duration is set to be long under the operating condition that the combustion of the air-fuel mixture progresses rapidly, knocking may occur in the spark discharge duration. Hence, there is fear that the knocking cannot be detected on the basis of the ionic current at the time of completion of the spark discharge.
Therefore, preferably, the ignition device for internal combustion engine may further have: a spark discharge duration calculation unit for calculating a spark discharge duration required for combustion of an air-fuel mixture by the spark discharge of the spark plug, on the basis of an operating state of the internal combustion engine; and a spark discharge interruption unit for forcibly interrupting the spark discharge of the spark plug in accordance with the spark discharge duration calculated by the spark discharge duration calculation unit.
In this manner, in the ignition device for internal combustion engine having the spark discharge interruption unit, the spark discharge completion timing is not fixed as the completion timing based on natural extinction but can be set to any timing in accordance with the operating state of the internal combustion engine. In addition, because the spark discharge is forcibly interrupted in accordance with the spark discharge duration calculated on the basis of the operating state of the internal combustion engine, knocking can be detected before extinction of the generated knocking even in the operating state in which the combustion of the air-fuel mixture progresses rapidly.
Because generation of ions accompanies combustion of the air-fuel mixture (fuel), the ion generation timing in an operating state in which the combustion of the air-fuel mixture progresses rapidly is earlier than that in an operating state in which the combustion of the air-fuel mixture progresses slowly. Accordingly, when the spark discharge is forcibly interrupted in accordance with the spark discharge duration calculated on the basis of the operating state of the internal combustion engine as shown in the invention, the timing of generation of knocking overlaps the timing of production of a large number of ions so that accuracy in detection of knocking can be improved more greatly.
For example, the spark discharge interruption unit may be preferably formed so that the spark discharge interruption unit forcibly interrupts the spark discharge of the spark plug by re-starting current conduction to the primary winding in accordance with the timing that the spark discharge duration has passed after the ignition switching unit turns off the current flowing in the primary winding of the ignition coil.
That is, generation of the spark discharge is performed by use of the principle of carrying a current to the primary winding of the ignition coil to induce magnetic flux and then interrupting the conduction of the current to change magnetic flux rapidly to induce a high voltage in the secondary winding of the ignition coil. When a current is carried to the primary winding once again while the spark discharge is generated, the direction of the change of the primary current flowing in the primary winding is reversed from a decreasing direction to an increasing direction. As a result, the direction of the change of magnetic flux in the ignition coil is reversed, so that the induced voltage generated between the opposite ends of the secondary winding is reduced. Because the induced voltage generated in the secondary winding is reduced by re-starting the current conduction into the primary winding in this manner, the voltage applied to the spark plug can be reduced to a value lower than the required value necessary for generation of the spark discharge.
That is, if the spark discharge interruption unit is formed so that the current conduction to the primary winding of the ignition coil is re-started, the voltage applied to the spark plug can be reduced to a value lower than the required value. As a result, the spark discharge in the spark plug can be forcibly interrupted.
Incidentally, when the spark discharge is forcibly interrupted, the detection timing control unit may start application of the ionic current-detecting voltage at the point of time when the detection delay time has passed after the forcible interruption timing as the starting point. On the other hand, when the spark discharge is not forcibly interrupted, application of the ionic current-detecting voltage may be started at the point of time when the detection delay time has passed after the natural extinction timing of the spark discharge.
Incidentally, in a recent central electronic control unit (ECU) for internal combustion engine, there are executed many control processes not only for ignition control but also for air-fuel ratio control, fuel injection timing control, etc. on the basis of input signals given from sensors (such as a crank angle sensor, an exhaust gas detection sensor, etc.) provided in respective parts of the internal combustion engine. Hence, load on internal processing by the ECU becomes considerably large. Therefore, when a unit for generating and detecting the ionic current is provided, it is preferable to design the unit so that the load on processing by the ECU does not increase.
Therefore, preferably, in the ignition device for internal combustion engine, the external commands are controlled by a switching drive unit for switching-controlling the ionic current detection switching unit on the basis of at least one of a duration of conduction of the primary current and the spark discharge duration.
That is, the ionic current detection switching unit can be switching-controlled without a new signal set in the ECU. Hence, the ionic current can be generated and detected well without increase of load on the ECU.